1. Field of the Invention
The present invention relates to a piezoelectric thin film resonator and a filter using the same, and particularly to an effective technology which is applied for preventing unnecessary acoustic coupling that occurs between the piezoelectric thin film resonators.
2. Related Background of the Invention
The demand for compact and low-loss RF filters has been growing along with the development of cell-phones. Further, due to the requests for high-speed and large-capacity communications, higher-frequency carrier waves have been progressively developing in the communication systems.
In response to such requests, the filters that use piezoelectric thin film resonators have been drawing increasing attention recently. The piezoelectric thin film resonator has a structure in which a piezoelectric thin film is interposed between upper and lower electrode films, and presents a predetermined resonance characteristic by applying a high frequency signal between the upper and lower electrode films. Such a filter that uses a piezoelectric thin film resonator has a structure in which the piezoelectric thin film resonators are electrically connected in a ladder shape.
In a conventional filter, a piezoelectric thin film resonator has a shape of an oblong or a square, that is, a shape of a rectangle. Note that a description of the shape of a piezoelectric thin film resonator can be found in 1995 IEEE ULTRASONICS SYMPOSIUM P905–908/SOLIDLY MOUNTED RESONATORS AND FILTERS, and especially in FIG. 4 therein.
In the present invention, “the shape of a piezoelectric thin film resonator” does not indicate an individual shape of a piezoelectric thin film or electrode films that constitute a piezoelectric thin film resonator, but indicates the shape of an area in which a piezoelectric thin film and electrode films having this piezoelectric thin film interposed therebetween overlap each other. In other words, the shape of a piezoelectric thin film resonator indicates an area in which a piezoelectric thin film and two electrode films having this piezoelectric thin film interposed therebetween overlap each other, when looking at the piezoelectric film and the two electrode films in a direction where they are arranged in a line.
Incidentally, regarding a filter, it is important that the interval between adjacent piezoelectric thin film resonators is designed to be as narrow as possible in order to achieve a low-loss by miniaturizing a chip and reducing the length of a wire.